1. Field of the Invention
The present invention relates to magnetic sensing devices which can strongly pin magnetization of pinned magnetic layers by uniaxial anisotropy of the pinned magnetic layers themselves.
2. Description of the Related Art
Magnetic sensing devices including multilayer materials each formed by laminating a free magnetic layer, a nonmagnetic material layer, and a pinned magnetic layer are classified into two types depending on the current direction with respect to the multilayer materials; i.e. a current-in-the-plane (CIP) type and a current-perpendicular-to-the-plane (CPP) type.
In the CIP-type magnetic sensing devices, current is applied to the multilayer materials in the direction parallel to the film surfaces. On the other hand, in the CPP-type magnetic sensing devices, current is applied in the direction perpendicular to the film surfaces of the multilayer materials.
The CPP-type magnetic sensing devices are assumed to be advantageous compared with the CIP-type magnetic sensing devices, because the CPP-type magnetic sensing devices can magnify reproduced outputs regardless of their reduced device sizes. Nowadays, the CPP-type magnetic sensing devices are expected to have a structure capable of responding to an increasing high recording density as a replacement of the CIP-type magnetic sensing devices, which are the current mainstream of the magnetic sensing devices.
Japanese Unexamined Patent Application Publication No. 2002-150512 (referred to as Patent Document 1 hereinafter) discloses a CPP-type magnetic sensing device, in particular, a structure enhancing spin-dependent scattering of conduction electrons and improving the sensitivity by forming a free magnetic layer or a pinned magnetic layer having a composite of a plurality of thin films and nonmagnetic layers alternately laminated.
Japanese Unexamined Patent Application Publication No. 8-7235 (referred to as Patent Document 2 hereinafter) discloses a system for pinning magnetization of a pinned magnetic layer by uniaxial anisotropy of the pinned magnetic layer itself.
In conventional magnetic sensing devices, the presence of very thick antiferromagnetic layers causes disadvantages, such as a decrease in a giant magnetoresistive (GMR) effect. The disadvantages in the decrease in the GMR effect will now be described referring to the CPP-type magnetic sensing device.
FIG. 13 schematically shows a structure of the conventional CPP-type magnetic sensing device. Specifically, the device has a multilayer material including a free magnetic layer 1, nonmagnetic material layers 2 on the top and bottom of the free magnetic layer 1, pinned magnetic layers 3 on both of the nonmagnetic material layers 2 so as to sandwich the free magnetic layer 1 and the nonmagnetic material layers 2, and antiferromagnetic layers 4 on both of the pinned magnetic layers 3. Electrodes 5 and 6 are further provided on the top and bottom of the multilayer material.
In the structure shown in FIG. 13, the pinned magnetic layers 3 each include a three-layer composite having two magnetic layers 3a and 3c and a nonmagnetic intermediate layer 3b disposed between the magnetic layers 3a and 3c. Magnetization of the magnetic layers 3a and 3c is antiparallel to each other. Such a composite is called an artificial ferri-structure.
For example, the free magnetic layer 1 is made of a Ni—Fe based alloy, the nonmagnetic material layers 2 are made of Cu, the magnetic layers 3a and 3c of the pinned magnetic layers 3 are made of a Co—Fe based alloy, the nonmagnetic intermediate layers 3b of the pinned magnetic layers 3 are made of Ru, and the antiferromagnetic layers 4 are made of a Pt—Mn alloy.
In the structure shown in FIG. 13, since the antiferromagnetic layers 4 have a high resistivity, e.g. a resistivity of about 200 μΩ·cm2 (or more), the antiferromagnetic layers 4 generate Joule heat when a current is applied between the electrodes 5 and 6. As Joule heat is generated in the antiferromagnetic layers 4, phonon scattering and electromigration caused by lattice vibration of conduction electrons are intensified in the adjoining pinned magnetic layers 3, the nonmagnetic material layers 2, and the free magnetic layer 1.
It is assumed that a resistance change per unit area (ΔR·A) of the CPP-type magnetic sensing device is closely connected to the effect of spin-dependent bulk scattering. In the structure shown in FIG. 13, the change in resistance (ΔR) is affected by the free magnetic layer 1 and the magnetic layers 3c, which are in contact with the nonmagnetic material layers 2, of the pinned magnetic layers 3. In particular, the magnetic layers 3c must have a positive coefficient of spin-dependent bulk scattering (β value) in order to allow up-spin conduction electrons to flow in the magnetic layers 3c and to allow down-spin conduction electrons to be scattered in the magnetic layers 3c. This increases differential spin-diffusion length between the up-spin conduction electrons and the down-spin conduction electrons, resulting in an increase of the resistance change per unit area (ΔR·A).
However, the above-mentioned phonon scattering by lattice vibration of the conduction electrons generates spin-independent scattering of the conduction electrons. As a result, the CPP-type magnetic sensing device cannot be sufficiently improved in the GMR effect which is typified by a resistance change per unit area (ΔR·A).
In the structure shown in FIG. 13, since the antiferromagnetic layers 4 are thick, the distance between the electrodes 5 and 6 is large. Therefore, the structure cannot appropriately respond to the increasing recording density of recording media.
These problems similarly occur in the magnetic sensing device disclosed in Patent Document 1 because the antiferromagnetic layer is basically included in the film configuration of the device.
The GMR effect can be sufficiently improved by removing the antiferromagnetic layers 4 from the multilayered structure. In such a structure, magnetization of the pinned magnetic layers must be sufficiently pinned by a means other than the antiferromagnetic layers 4.
With reference to Patent Document 2, magnetization of the pinned magnetic layers is pinned by uniaxial anisotropy of the pinned magnetic layers themselves, instead of disposing the antiferromagnetic layers.
In the structure disclosed in Patent Document 2, a pinned ferromagnetic layer (pinned magnetic layer) is laminated on a buffer layer that is made of Ta and functions as a base material. Ta tends to be amorphous and has a high resistivity. Therefore, when this structure is applied to a CPP-type magnetic sensing device, the buffer layer generates heat in the similar manner to the conventional antiferromagnetic layer and spin-independent scattering of the conduction electrons occurs. Consequently, it is assumed that the GMR effect cannot be sufficiently improved. Furthermore, Patent Document 2 does not sufficiently disclose how magnetization of the pinned ferromagnetic layer is strongly pinned by using the buffer layer made of Ta.
In Patent Document 2, though a first ferromagnetic film is distant from the buffer layer, “self pinning” of the first ferromagnetic film is not performed. Furthermore, in Patent Document 2, the magnetization of the first ferromagnetic film and a second ferromagnetic film is antiparallel to each other and in an unstable state to the outer magnetic field.
However, any modification of an interface structure between the first ferromagnetic film and a spacer layer made of Cu should be avoided because such modification causes a decrease in the GMR effect. Therefore, the issue for strongly pinning magnetization of the first ferromagnetic film cannot be solved.